Floods in Ouagadougou have almost brought the city to a standstill
Heavy flooding has now affected some 350,000 people across West Africa, killing at least 32 in Ghana and Burkina Faso, UN officials say.
More than 150,000 people in Burkina Faso have fled their homes, mainly in the capital Ouagadougou.
A UN spokeswoman said the amount of rain that fell in Ouagadougou on one day this week was equal to a quarter of the whole country's annual rainfall.
Neighbouring countries affected include Benin, Guinea, Niger and Senegal.
The United Nations is warning that numbers killed are likely to rise and has sent an assessment team to Burkina Faso.
At least seven people are known to have died in Burkina Faso, where the capital has seen the heaviest rains in 90 years.
The BBC's West Africa correspondent John James says some of the displaced have moved in with friends and families, but most are dependent on the government which is struggling to cope.
The Burkinabe authorities say there is a desperate need for blankets, clothes and food and President Blaise Compaore has appealed to the international community for help.
He said those made homeless were being temporarily rehoused in schools and colleges.
The waters flooded 24,000 homes and the city's infrastructure - roads have been washed away and electricity has been cut off.
Prime Minister Tertius Zongo said on Friday the cost of flood damage had risen to $152m (£92m) and included two destroyed dams and 12 damaged bridges.
Earlier in the week, Ouagadougou's main hospital was badly flooded from water pouring over a nearby dam.
The Belgian Red Cross says 60 children as well as other patients had to be evacuated.
The authorities were forced to open the main gate of a hydroelectric dam in the Volta River basin near the Ghana border to prevent further flooding.
"Cultivated land on the reservoir's shores and further upstream will be flooded. We warn riverside residents to stay away from the shores," Venance Bouda, the official in charge of hydroelectric power, told the Associated Press news agency.
In Ghana at least 25 people have already died in flooding-related incidents, officials say.
The torrential rains have also hit other countries in the region - in Senegal 30,000 homes were flooded while in Guinea 20,000 people are affected.
Our reporter says similar floods in 2007 killed 300 people.